Bubble formation is required in a variety of different fields and is a well studied and reported area of research. Recently, attention has been paid to various methods of utilising fine bubbles having a diameter of micrometer level and various apparatus for generating fine bubbles have been proposed. Fine bubbles are particularly advantageous as, for a given volume of gas, more smaller bubbles form a greater surface area than fewer larger bubbles.
Some of the common techniques used to form gas bubbles include: compressed air to dissolve air into a liquid stream, which is then released through nozzles to form bubbles by cavitation; air streams delivered under a liquid surface, where bubbles are broken off mechanically, optionally by agitation or shear forces; and ultrasonic induced cavitation.
In a system to generate air bubbles by introducing air into water flow with a shearing force using vanes and an air bubble jet stream, it is often required to employ a higher number of revolutions to generate cavitation. However, problems arise such as power consumption increase, corrosion of vanes or vibrations caused by the generation of cavitation. Further, such a technique does not lend itself to generating large amounts of fine bubbles.
The desire for small bubbles is that they provide a variety of excellent effects, which have been utilized in many industrial fields including plant cultivation, aquafarming, wastewater treatment and the like. It is effective to reduce the diameter of bubbles to increase their surface area relative to their volumes, thereby enlarging the contact area between the bubbles and the surrounding liquid. Thus, a more rapid mass transfer process can take place when the bubble size is reduced.
In wastewater treatment plants, it is known to aerate effluent, or sludge, as part of the wastewater purification process. Generally, air is introduced near the bottom of an aeration tank containing wastewater and bacterial floc via a system of pipes and/or hoses. As the air rises to the surface as air bubbles, some of the oxygen in the air is transferred to the wastewater and is consumed by the respiring bacteria during digestion, which aids in the treatment of sewage. The more oxygen that is supplied to the bacteria, the more efficient the digestion process. It is desirable, therefore, to provide smaller bubbles whereby to enhance further the efficiency of the digestion process.
A similar requirement exists in bioreactors and fermenters in cases where they are sparged for aeration purposes. Specifically, the yeast manufacturing industry has the requirement where growing and reproducing yeast bacteria need constant oxygen replenishment for respiration purposes.
However, in an aeration system using a conventional-type fine bubble generating system (such as a diffusion system based on injection), when air bubbles are injected under pressure through pores, the volume of each bubble is expanded and the diameter of each bubble is increased to several millimetres due to the surface tension of the air bubbles during injection, even when fine pores are provided. This method therefore encounters difficulty in generating fine bubbles of small diameter. Another problem associated with such a method is the clogging of the pores, which reduces the efficiency of the system.
A further application of small bubbles is the extraction of hard-to-lift oil reserves in some fields which either have little oil left, or have the oil locked in sand. Bubbling gas up through such oil-bearing reserves has the effect of lifting the oil as the bubbles rise under gravity and bring the oil with them. The bubbles are formed in water and pumped into the well or reserve and the oil is carried at the interface between the gas and water of each bubble as it passes through the reserves. Hence, the smaller the bubble, the greater the relative surface area for transporting the oil.
Bubbles are also used in fuel cells that utilise a liquid catalyst/mediator system. In some of these systems, oxidation of a redox couple after it has been reduced at the cathode is required. In order to do this, an oxidant such as oxygen or air is passed through the redox couple solution in the form of bubbles.
Given sufficient availability of oxygen in solution, this oxidation reaction is suggested to be rapid (Atherton et al., 2010). However, in practice, the overall reaction rate is limited by the transfer of oxygen into solution. The rate of oxygen mass transfer is directly proportional to the interfacial area between the gas and liquid phases. Small bubbles have a large collective interfacial area, thereby improving oxygen mass transfer to the liquid.
Fuel cells have numerous applications, many of which (such as in the automotive industry) require the fuel cell to have a small volume and a short residence time, as well as low power consumption. The preferred gas/liquid contacting method involves air sparging into a fast moving liquid stream, within a regenerator region. The volume of the fuel cell is partially dependent on the necessary size of the regenerator region, in which the redox couple is oxidised. This in turn is dependent upon the creation of a large interfacial area within a small volume, to maximise the oxygen mass transfer. Key to achieving this is the generation of small bubbles.
In certain applications, bubbles form on the surface of the device past which the liquid flows. The flow of liquid provides a shearing force on the bubbles, thereby prematurely shearing them from the porous surface before they are allowed to inflate to a larger size.
It is thus desirable to generate fine bubbles in a more convenient and efficient manner than known hitherto. Also important are durability and energy efficiency. To be of use in a number of applications, the device must incur a sufficiently low parasitic load and maintain performance over many hours (for example, around 10,000 hours with numerous start-stop cycles would be required for an automotive fuel cell).
The general perception is that in order to reduce the size of a bubble, the solitary requirement is for the pore size through which the bubble is formed to be reduced. However, there are a number of reasons why this perception is ill-conceived.
The first of these reasons is that the bubble is “anchored” to the substrate material through which it is formed, and will continue to inflate until the bubble breaks free by some disruptive force. The forces can, for instance, be buoyancy, inertial or shear forces applied to the bubble as it develops. The interfacial tension controls the force with which the bubble is held by virtue of it being anchored to the surface. In this way, there are three interactions that need be considered:                the interaction between the liquid and solid substrate;        the interaction between the liquid and gas; and        the interaction between the solid substrate and gas.        
It has therefore been found that having an active surface on which bubbles are formed that attracts the liquid phase, for example a hydrophilic surface in the case of an aqueous liquid, is advantageous in producing small bubbles as the liquid favourably flows under the forming bubble and lifts it from the surface, thereby enhancing fine bubble generation.
Porous stainless steel sinter materials are known in industry as possible materials on which bubbles may form. Manufactured by the sintering of a fine steel powder of varied size and shape particles, the material has an inherent degree of randomness and non-uniformity. This is reflected in the size and shape of surface pores, as well as the size, tortuosity and interconnectivity of internal channels. The result is the existence of some “dead ended” channels and others which join significant distances laterally though the porous medium. This provides potential for uneven gas distribution across the porous surface. These characteristics are not terribly surprising considering that the material is developed primarily for the air filtration market (Grade 2 Mott is specified as excluding 99.9% of particles >2 mm in diameter).
However, the high degree of tortuosity can be beneficial as it restricts gas flow to the pores, generating a choked flow. This limits the rate of bubble growth and thus allows opportunity for them to be sheared from the surface at a smaller size than would be otherwise achieved. Bubble growth increases exponentially after reaching the diameter of the pore (i.e. after forming a hemisphere—see WO2011/107795 A2). If unchecked, this explosive growth can exceed the micro scale within a matter of nanoseconds. Hence, an effort to choke the ingress of gas into the bubble is desirable.
The fine surface pores (30-70 μm) of the sintered material do promote the generation of small bubbles. However, due to the range of pore sizes, their irregular shapes and random surface interconnectivity, it is the larger pores which tend to preferentially form bubbles. The formation of smaller bubbles from smaller pores requires more energy and so air flow will tend to divert and utilise the larger surface pores.
Although reasonable rates of oxidation are achievable when using this device in a fuel cell, inconsistent performance and a lack of durability are significant issues. Gas distribution across the porous surface often becomes uneven, resulting in the localised production of large bubbles, leading to significant variation in gas-liquid ratio throughout the contactor and a decline in available interfacial area. This culminates in poor oxidation performance and was observed to worsen with time.
The formation of fine bubbles is promoted by the high surface energy of the steel substrate and as such, the low contact angle between the liquid and the solid surface. However, it has been found that, combined with its fine interconnected pore structure, this property unfortunately means the material will very effectively wick liquid into its bulk with an estimated capillary pressure of about 300 mmH2O. This can occur when gas flow is halted, either intentionally stopped as part of normal operation or while in operation via unutilised pores. Due to the additional pressure drop incurred by the gas when attempting to expel liquid, further pores may also become unutilised (i.e. blocked) as flow diverts to less impeded pathways. The consequence of this is a change in gas distribution and therefore, bubble size distribution. Gas distribution across the porous surface can become so uneven that a stable gas-liquid dispersion is no longer achievable. The issue is further exacerbated by the dehydration of back soaked solution within the porous bulk. It is hypothesised that these conditions (i.e. dried redox couple salts held at −80° C. in the case of a fuel cell) are favourable to the formation of insoluble deposits which accelerate the rate at which the pores become blocked.
Accordingly, a more durable device with more uniform fine bubble formation is required. The present invention has been devised with this in mind.